1. Technical Field
The present invention is directed to a process for device fabrication. The present invention relates to that aspect of the process in which the device substrate is cleaned during fabrication.
2. Art Background
In device fabrication, it is becoming increasingly important to remove impurities such as particles, contaminated films, and adsorbed gases from the surface of the substrate during device fabrication. One way in which impurities are removed from substrates is by rinsing the substrate with a solution containing deionized water, ammonium hydroxide, and hydrogen peroxide. As observed in Kern, W., "The Evolution of Silicon Wafer Cleaning Technology," J. Electrochem. Soc., 137(6):1887 (1990), the substrate is rinsed with this solution using many different techniques such as immersion in a bath, spray cleaning and megasonic cleaning. A bath containing such a solution is described in U.S. Pat. No. 5,167,756 to Nakashima et al.
As observed in M. Meuris, et al., "The Relationship of the Silicon Surface Roughness and Gate Oxide Integrity in NH.sub.4 OH/H.sub.2 O.sub.2 Mixtures," Jpn. J. Appl. phys., 31 (Part 2, No. 11A): L1514 (1992) a small variation in the amount of ammonium hydroxide in the bath will affect both the efficiency with which the bath cleans semiconductor substrates and capacitor yield on those substrates. Furthermore, as observed in Kobayashi, H., et al., "Study of Si Etch Rate in Various Compositions of SC1 Solution," Jpn. J. Appl. phys., 32(Pt. 2, No. 1A/B):L45 (1992) the concentrations of ammonium hydroxide and hydrogen peroxide affect the rate at which the cleaning solution etches the semiconductor substrate.
A standard cleaning solution has an ammonium hydroxide concentration of about 0.01 to about 10 percent by weight of cleaning solution and a hydrogen peroxide concentration of about 0.01 to about 10 percent by weight of cleaning solution. However, ammonia evaporates from the solution and hydrogen peroxide decomposes over time and, therefore, the concentrations of ammonium hydroxide and hydrogen peroxide in the cleaning solution are not constant over time. Eventually, the concentrations of ammonium hydroxide and hydrogen peroxide change so much that the solution is no longer useful for cleaning substrates. Substrates such as silicon wafers, flat panels, and photomasks are therefore typically cleaned by a batch process in which the solution is discarded when it is no longer useful because the concentrations of ammonium hydroxide or hydrogen peroxide are no longer within the desired range.
Although this batch approach has high costs associated with reduced production throughput (because the time it takes to change the solution is time in which substrates are not being cleaned) and chemical use and disposal, there is currently no commercial process for controlling the concentration of ammonium hydroxide and hydrogen peroxide in the bath over time as the bath is used to clean substrates. Although control schemes are described in Japanese Patent Application No. SHO 59[1984]-46032 to Harasono et al. and Japanese Patent Application No. SHO 62[1987]-8040 to Miyabi et al., these control schemes do not control both the concentration of ammonium hydroxide and the concentration of hydrogen peroxide in the cleaning solution and therefore do not provide a mechanism for significantly extending the useful life of the bath. A process for cleaning substrates in which the concentrations of both ammonium hydroxide and hydrogen peroxide are controlled such that they are maintained within a desired concentration range as the bath is used to clean substrates is therefore desired.